Geotechnical Issues for Developing Coastal Waste Landfills

Abstract

Coastal landfilling has become one of the most important practices in the disposal of municipal solid waste incinerator ash (MSWIA) as well as industrial waste generated in megacities in Japan. Leachate generated from these waste materials may contain substances, particularly heavy metals, that are harmful to the environment. Mechanical properties of coastal waste landfills have not been comprehensively studied. Considering the post-closure use of landfill sites, both mechanical properties and long-term mobility of heavy metals are important geotechnical issues. This article addresses the results of large-scale column percolation and modified batch tests conducted to evaluate the mobility of heavy metals in coastal MSWIA landfills, based on heavy metal speciation using a sequential extraction method. The column percolation test was conducted to simulate the behavior of zinc (Zn), as a trace metal, in the waste-bottom marine clay system. Modified batch tests were employed to investigate the effects of pH and Eh on the forms of Zn in both MSWIA-leachate and leachate-marine clay systems. Zn was effectively immobilized by forming exchangeable and reducible fractions under moderately alkaline conditions or reducible and oxidizable fractions under highly alkaline conditions. However, Zn mobilization under neutral conditions was involved, since the formation of exchangeable compounds more predominantly contributed to immobilization. Test results on metal speciation in the marine clay-leachate system revealed that marine clay acted as an effective attenuation layer. These findings support the premise that Zn mobility is limited in the coastal MSWIA landfills. Secondly, a series of triaxial consolidated undrained (CU) compression tests were carried out on reconstituted waste samples before and after being cured in simulated coastal landfill leachate water for different periods in order to understand the aging effects on the mechanical properties of the landfilled waste layer. Peak shear strength and deformation modulus increased through curing. However, residual strength was not affected by differences in the curing periods. The waste mixture layer investigated in this study could possibly be used as a foundation layer with sufficient bearing capacity in post-closure use of coastal landfill sites.